Resolution thermal spirit masters method

ABSTRACT

Thermal transfer processes are improved by irradiating an original transfer sheet image receiving sheet with discrete bundles of radiation energy.

United States Patent H 1 1 111 3,

Gaynor [45] Dec. 9, 1975 (54] RESOLUTION THERMAL SPIRIT MASTERS 3,010,389 1 1/1961 Buskes 101/467 X METHOD 3,414,724 12/1968 Kvarnegard 250/318 3,666,465 5/1972 Winnek 96/45 [75] Inventor: Joseph Gaynor, Arcadia, Calif. 3 79x 3e,5 3/1974 Browning et a1 178/(16 A 173| Assi nee: Bell & Howell Com an Ch'cz .1

g L p y FOREIGN PA'l'EN'lS 0R APPLICAHONS 1 96 35 I22 F lcd: July 23 1973 741,69 12/1955 UIHLLtl Kingdom 21 A N 3 e v l I 0 8| Irmzury lz.\umlm r(lyde l, Coughenour Related [1.8. Application Data Attorney, Agcnl, or l"irmRobert A. Walsh; (Jerald B. [621 Division of Scr. No 130.722. April 2, 1971, P

abandoned.

152] 11.8. (1 101/467; 101/471 57 ABSTRACT [51] Int. Cl. .1 841m 5/18;B41m5/26;B4lcl/1(1 1 1 Field of Search 101/467, 470, 47l; Thermal transler processes are improved by irradiat- 1 3|8, 31 ing an original transfer sheet image receiving sheet with discrete bundles of radiation energy. l56| References Cited UNITED STATES PATENTS l,Claim, 3 Drawing Figures 114491136 3/1912 Ernst 96/45 BACKGROUND OF THE INVENTION This invention relates to duplicating processes and more particularly to duplicating processes which at some stage utilize a thermal transfer process.

Briefly, thermal transfer processes comprehend transferring a portion of a transfer coating, which corresponds either mirror-wise or image-wise to an original image, to an image receiving sheet. The image receiving sheet may be a master from which subsequent copies are produced as in the spirit duplicating process or it may constitute the ultimate copy.

The myriad of duplicating processes which at some stage include a thermal transfer process and the large number of ways to accomplish such a process are fully disclosed in US. Pat. No. 3,122,998, issued Mar. 3, 1964 to Raczynski et al. Since all of those thermal processes have common difficulties which are minimized by this invention, it will be described only in conjunction with the thermally prepared spirit duplicating master although it is understood that it is applicable to all such processes as exemplified by the aforementioned patent.

As is well known, the resolution and sharpness of spirit copies is low, particularly those copies produced from thermally prepared spirit masters. The fundamental reason for this lack of resolution in the thermal spirit master is that the heat preferrentially generated in the black parts of the original must be transported both through a relatively thin film and also through a dyewax layer which comprise the transfer sheet. The heated dye-wax layer then melts or plasticizes and transfers to another substrate called the master sheet.

Heat does not travel unidirectionally, but in all directions simultaneously. As a result, image resolution is degraded by lateral thermal diffusion.

An additional contributory problem derives from the fact that large and small black areas in the original transfer their heat to the immediate surroundings at different rates. Therefore, their dynamic temperatures at any time are not the same. As a result, the exposure required to obtain a reasonably good thermal master image for a large area is significantly different and lower than the exposure required to obtain a thermal master in a smaller area. Since originals usually contain image areas which are large and small, this means that some intermediate setting or exposure time, optimum for neither one, is required to obtain an adequate master.

SUMMARY OF THE INVENTION The problems of lateral conductivity and varied exposure requirements are minimized by this invention. Means are provided for exposing the transfer assembly to highly concentrated radiant energy in a predeter mined pattern as opposed to conventional uniform irradiation.

In one form of the invention radiant energy is focused through an assembly of minute lenses that produce lines or dots of highly concentrated radiant energy. The spacing of the focused energy is maintained two to four times less than that which produces the resolution desired in the thermally transferred image.

2 Byso concentrating the radiant energy the exposures required for large and small black image areas become virtually equal. .Additionally, the rapid infusion of energy reduces exposure time and thereby lessens lateral heat transfer.

BRIEF DESCRIPTION THE DRAWINGS The features of this invention will become more apparent in the following detailed description which is to be read in conjunction with the drawings in which:

FIG. 1 is a schematic illustration of an exposure device incorporating the principles of this invention.

FIG. 2.is a bottom view of the lenticulated cylinder shown in FIG. 1. l

FIG. 3 is an alternative embodiment of a cylinder incorporating the principles of this invention.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates a preferred embodiment of this invention. As with conventional thermal transfer processes a sandwich has been formed of an original 10, a thermal transfer sheet 12 and an image receiving sheet 14. The thermal transfer sheet 12 com prises a supporting sheet 16 and a transfer coating 18.

Typically the supporting sheet 16 may be made of Mylar and the transfer coating 18 may be a dye-wax mixture. Reference is again made to US. Pat. No. 3,122,998 wherein many acceptable materials are listed.

The sandwich is compressed between a pressure roller 20 and a transparent drum 22 to insure intimate contact between the transfer coating 18 and the image receiving sheet 14, which may be thought of as a spirit duplicator master.

Radiant energy from a source 30 passes through the drum 22 and impinges on the original 10. As is well known in the art the dark or imaged areas of the original 10 selectively absorb the radiant energy and thus heat to a higher temperature than the non imaged areas for the same exposure. The exposure time is selected to permit the conduction of heat from the imaged areas of the transfer coating 18 causing it to soften to the point that it will transfer to the image receiving sheet 14. Upon separating the image receiving sheet 14 from the assemblage it will carry portions of the transfer coating 18 corresponding to the mirror image of the original 10.

In the past, it has been the practice to uniformly irradiate the assembly through the transparent drum 22. But, in accordance with this invention a plurality of lenticular lenses 24 have been formed on the outer surface of the drum 22. As shown herein the lenses 24 are grossly exaggerated in size.

Preferably the lenses 24 have a focal length equal to the thicknesss of the assembly such that the radiant energy is focused at the imaged surface of the original 10 as at 26. The assembly instead of being uniformly irradiated will be irradiated by a plurality of discrete bundles of energy as dictated by the lens shape.

For example, if the lenses 24 extend across the width of the drum 22 as depicted in FIG. 2 the energy bundles would be spaced parallel lines. On the other hand, if the lenses 24' were circular as shown in FIG. 3 the energy bundles would impinge the original 10 in the form of spaced points. Many other ways and patterns of concentrating the radiation would be apparent to one of ordinary skill in the art.

It is important that the spacing between the concentrated energy does not exceed the normal human resolution capabilities or that spacing would become noticeable in the transferred image. In general, the human eye cannot resolve spacing any greater than about five line pairs per millimeter. In a practical sense, it would be preferable to maintain this spacing closer than that and Illline pairs per millimeter would be a good practical spacing to maintain.

Thus, an improved copying method and apparatus has been described. It is not intended to limit this invention to the preferred embodiment disclosed but rather it is to extend to the many modifications and equivalents of the invention as set forth in the appended claims.

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows:

4 I. A method for thermally transferring an original image comprising the steps of:

forming an assemblage of an original image, a thermal transfer sheet and an image receiving sheet;

simultaneously applying pressure to said assemblage using a drum transparent to radiant energy surrounding a radiation source and having a plurality of lenticular lenses perimetrically disposed thereon, and irradiating said original image with a plurality of concentrated spaced bundles of thermal energy focused thereon from said source using said lenses, thereby lessening lateral heat transfer and providing variable optimum exposure time;

ceasing said pressure application and said irradiation;

and

separating said image receiving sheet from said assemblage. 

1. A method for thermally transferring an original image comprising the steps of: forming an assemblage of an original image, a thermal transfer sheet and an image receiving sheet; simultaneously applying pressure to said assemblage using a drum transparent to radiant energy surrounding a radiation source and having a plurality of lenticular lenses perimetrically disposed thereon, and irradiating said original image with a plurality of concentrated spaced bundles of thermal energy focused thereon from said source using said lenses, thereby lessening lateral heat transfer and providing variable optimum exposure time; ceasing said pressure application and said irradiation; and separating said image receiving sheet from said assemblage. 